Demonstrators tried to march to the electoral board's downtown Caracas headquarters but were turned back by long lines of police in riot gear.

Tuesday's protest is at least the fourth in as many weeks. Each time, police have stopped protesters from reaching downtown.

The government began a tentative dialogue process with the opposition last month, an effort mediated by former Spanish Prime Minister Jose Luis Rodriguez Zapatero and the former presidents of Panama and the Dominican Republic.

On Tuesday, the Union of South American Nations, which is backing that mediation effort, said the opposition had not shown up to the most recent meeting in the Dominican Republic. They called on Maduro's critics to return to the negotiating table as soon as possible, calling it the "only way to reaffirm peace, coexistence and dialogue as a tool for mutual understanding among Venezuelans."

Many in the opposition are convinced the government is trying to buy time amid growing international pressure and has no intention of meeting its demands to set a date this year for the referendum or to free dozens of jailed opponents it considers political prisoners. The Secretary General of the Organization of American States has called for special proceedings to evaluate Venezuela's commitment to democracy and the rule of law, a process that could end with the country's suspension from the hemispheric group.

"What we're seeing today, this deployment of troops, is an example of the government's dialogue," said opposition leader Henrique Capriles after a group he led was turned back with tear gas.

But some say the street pressure may be bearing fruit.

Late Tuesday, the head of the opposition alliance announced that electoral authorities had validated 1.3 million signatures on petitions demanding a recall referendum be held. After a meeting with electoral authorities Jesus Torrealba said that details of the next phase of the petition drive, demanding the collection of more than 4 million signatures, would be announced on Wednesday.